It is sometimes desirable to place into a subterranean wellbore, temporarily or permanently, mechanical or electrical downhole equipment which possesses, induces or provides a magnetic field with a stronger intensity than that of the earth's natural magnetic field. Examples of this equipment include, but are not limited to, MRI tools, solenoid actuators, and magnetic couplings. Magnetic couplings might be used to operate downhole safety valves, sliding sleeves, adjustable chokes, and pumps.
It is also sometimes desirable to use certain magnetic or electric downhole tools which may be affected or whose operation may be impeded by the presence of strong magnetic or electric fields. Examples of such downhole tools include but are not limited to subterranean logging devices, flow meters, formation evaluation tools, directional drilling equipment, directional or other survey instruments, coils, gyroscopic apparatus, MRI tools, photo-multipliers, casing or tubing collar locators, information gathering and/or transmitting devices and various electrical tools.
An objective of this invention is to reduce or minimize the interference of subterranean electric or magnetic fields with the efficient or effective operation of downhole tools.